A coaxial cable is usually comprised of an inner conductive member, a dielectric system surrounding the inner conductor, and an outer conductive member coaxially surrounding the dielectric system. The inner conductive member and the outer conductive members are made with some appropriate metal, most commonly copper, aluminum or some alloy containing such metal. The dielectric system is usually composed of some suitable plastic, and use of polyethylene, polystyrene, and polypropylene, in expanded or unexpanded form, is common.
The best dielectric, from a theoretical standpoint, would be a layer of air, which has a dielectric constant of 1.0. It is virtually impossible to construct such a cable, however, and commercial cables employ solid materials with necessarily higher dielectric constants. The higher the dielectric constant of the material, the lower the velocity of propagation of the coaxial cable as a whole, and thus, the longer the cable will take to transmit an electrical signal along its length. In addition to improved velocity of propagation, a lower dielectric constant will allow a thinner insulation layer which should produce a smaller finished cable diameter. This becomes important in applications which have space or weight limitations.
One method which has been followed in attempting to increase the velocity of propagation of a cable has been to decrease the effective dielectric constant by introducing air or other materials into an otherwise solid dielectric layer.
In U.S. Pat. No. 3,309,458, a coaxial conductor is shown which employs as a dielectric a two-layer system. The first layer of the system is comprised of a brittle foamed synthetic resin and the second layer is composed of a nonfoamed synthetic resin which is pliable in comparison with the foamed resin.
In U.S. Pat. No. 3,573,976, a coaxial cable is provided in which the dielectric is extruded from a combination of glass, silica or ceramic microspheres; a suspension of powdered polyethylene or polymeric fluorocarbon resin; a volatile ethylene dichloride or trichloroethylene carrier and a tackifying agent of polyisobutylene or hexafluoropropylenevinylidene fluoride copolymer. The microspheres, or microballoons as they are also known, are discrete, hollow, spherical particles, and the effective dielectric constant of the dielectric system is reduced according to the amount of air encapsulated therein.
U.S. Pat. No. 3,968,463 discloses a coaxial cable having as a dielectric coating on the core conductor, an extruded cellular ethylene or propylene polymer based composition.
U.S. Pat. No. 4,107,354 is directed to a method of forming a coaxial cable by coating a center conductor of the cable with a dielectric composed of cellular polyolefin.
The problem which has been encountered with coaxial cables employing foamed dielectric systems is that as the amount of foaming, and therefore the amount of encapsulated air, is increased, the mechanical and heat resistance properties of the cable are adversely affected. To provide sufficient mechanical strength, cables must have diminished flexibility or increased size, and this limits the applications for which the cable may be used.
Another method used to incorporate air into the dielectric system has been through the use of disk type insulating separators. Following this method, disk type insulating separators of a material such as polyethylene are fitted onto an inner conductor at spaced intervals, thereby leaving air filled interstitial spaces. Such construction, however, lacks mechanical strength, particularly when the coaxial cable is bent, and the cables must be handled with great care.
It is an object of the present invention to provide a dielectric system for a coaxial cable which has a low effective dielectric constant.
It is a further object of the present invention to provide a dielectric system for a coaxial cable which has a low effective dielectric constant, but which has sufficient mechanical strength to allow substantial flexibility in the finished cable.
It is still a further object of the present invention to provide a dielectric system for a coaxial cable which has a low effective dielectric constant, but which has sufficient mechanical strength over a substantial range of temperatures to allow the construction of cables of very small diameter with consistent and predictable electrical characteristics, which are particularly useful in applications which call for miniaturized electrical conductors.
The foregoing, as well as other objects, features, and advantages of the present invention are pointed out with particularity in the claims annexed to this specification. Further, they will become more apparent in light of the following detailed description of the preferred embodiment thereof and as illustrated in the accompanying drawings.
According to the present invention, there is provided a dielectric system for coaxial electrical conductors which separates an inner and outer conductive material. The dielectric system of the present invention comprises a first layer of braided high tensile strength polymeric fluorocarbon filaments in an open weave surrounding an inner conductor along its length. This layer of braided filaments is in turn covered by a second layer consisting of cellular polyparabanic acid tape which is helically wound along the length of the cable with a negative overlap. Circumferentially surrounding these two layers along the length of the cable is a third layer consisting of polymeric film, which provides a continuous skin over the cellular polyparabanic acid layer. A fourth layer, consisting of a crosslinkable polymeric lacquer, surrounds the third layer and provides a continuous skin enclosing the dielectric system circumferentially.